1. Field of the Invention
The present invention relates to an illumination optical system and an illumination lens element, and in particular, relates thereto suitable for an endoscope for illuminating an object through a fiber-optic lightguide.
2. Description of the Related Art
In an illumination optical system, e.g., in an illumination optical system for an endoscope, in which a directional bundle of light rays, emitted from a surface secondary light source (luminous body), such as a fiber-optic lightguide, is diffused through an illumination lens element, what should be considered is how to diffuse the directional bundle of light rays uniformly toward the periphery of an area to be illuminated so that a uniform light-quantity distribution is obtained.
In conventional illumination optical systems and illumination lens elements (a diffusion plate), various attempts have been made to improve the quality and performance thereof; however, no satisfactory results have been attained. In other words, an illumination lens element constituted by, for example, a planoconvex lens element with a spherical surface or an aspherical surface, or constituted by a negative meniscus lens element, cannot attain uniform illumination, nor avoid the occurrence of eclipse on the peripheral area of the lens element. Furthermore, an illumination optical system formed by a plurality of lens elements causes an increase of the entire length of the optical system. Accordingly, if the optical system is applied to an endoscope, such a lengthy optical system causes adverse influence to the bending-operability of the endoscope. Still further, such a lengthy endoscope with a lot of lens elements causes a production cost increase.
In connection with the above, for example, Japanese Unexamined Patent Publication No. Hei-7-43620 has taught an illumination lens element having a surface formed like saw teeth; however, through such a serrated surface, the directions of light rays tend to be uniform, so that sufficient diffusion cannot be attained.
It is an object of the present invention to provide an illumination optical system and an illumination lens element, by which a bundle of light rays emitted from a luminous body is uniformly diffused so that a uniform light-quantity distribution is obtained.
The present invention is directed to an illumination optical system including a luminous body and an illumination lens element which diffuses light rays emitted from the luminous body, and is directed to the illumination lens element itself.
More concretely, in order to achieve the above-mentioned object, the illumination lens element has a rippled surface, constituted by peaks and troughs which are alternately provided, on at least one of the incident surface and the exit surface thereof. The rippled surface is formed as a curved surface whose cross-sectional shape is defined by a periodic function which can be differentiated, or is formed by a plurality of straight lines which approximate the above curved surface which can be differentiated.
In the case where the cross-sectional shape of the rippled surface, i.e., the peaks and troughs, formed on one of the incident surface and the exit surface is a periodic function which can be differentiated, there are countless tangential directions in an area along a predetermined direction. As a result, a bundle of light rays emitted from the exit surface can be uniformly diffused, so that a more uniform light-quantity distribution (characteristics of luminous intensity distribution) can be obtained. The reason why a shape formed by a plurality of straight lines approximating the curved surface which can be differentiated is referred to is that the manufacturing of the illumination lens element is considered. In other words, the rippled surface can be defined by a periodic function at the design stage; however, in the production stage, curved lines defined by the periodic function can be practically replaced with a lot of straight lines when data on the rippled surface is input to machinery.
The illumination lens element can be formed rotationally symmetrical with respect to an axis parallel with the center axis of a bundle of fibers (hereinafter, the optical axis). The above explained rippled surface is formed by a single periodic function; however, it is also possible to form the same by the combination of plural periodic functions.
The features of the present invention can be described as follows: the above explained peaks and troughs which are alternately provided are formed so that at least three or more tangential directions exist in one period of the function. In other words, the three or more tangential directions mean that the randomness of the directions of light rays being emitted from the illumination lens element can be increased. Accordingly, a more uniform light-quantity distribution (characteristics of luminous intensity distribution) can be expected.
The peaks and troughs formed on the illumination lens element can be described as follows:
(i) at least one of the incident surface and the exit surface is provided with the peaks and troughs which are alternately provided thereon;
(ii) in the case where an angle, formed by a tangential line with respect to any point on the peaks-and-troughs surface and an imaginary line orthogonal to the optical axis, is defined as an inclination angle, the inclination angle is to be distributed over a range from 0xc2x0 to xc2x160xc2x0 or more; and
(iii) when the peaks-and-troughs surface for one period is equidistantly divided by sufficient numbers of divisional points, a relation in which the larger the inclination angle is the more divisional points exist is established. In addition to the above, when the peaks and troughs are made periodic, there is an advantage, i.e., by varying the diameter of the illumination lens element, the same characteristics of luminous intensity distribution can be obtained with respect to the lightguides having different diameters.
As explained, the peaks and troughs are formed in a manner that the larger the inclination angle is the more divisional points exist, under the condition that the peaks-and-troughs surface for one period is equidistantly divided by sufficient numbers of divisional points. This is generally understood that the randomness of the directions of light rays emitted from the illumination lens element can be increased. Accordingly, a more uniform characteristics of luminous intensity distribution can be expected.
The illumination optical system and the illumination lens element according to the present invention, which are preferably used for a fiber-optic lightguide for transmitting light rays from a light source, can also be used for other purposes.
The present disclosure relates to subject matter contained in Japanese Patent Application No.2000-98518 (filed on Mar. 31, 2000) which is expressly incorporated herein by reference in its entirety.